i am not sure if i have a magic number for max, but at high temps it is important how LONG you hold it there. seeing a quick jump to 1500 is not the end of the world if it's only there for a few seconds. heat soak in the turbo is the worst enemy and only extended time at high temps will cause that. if you hold it at 1250 for 5 minutes, that is a reason for concern.
we need an answer from the turbo mfr's to get a solid answer.
jim